Idea for a dedicated Rhodes preamp, need help !

[PermO]

I understand the frequency roll off is the result of phase cancelation.

Playing with the calculator was very insightfull seeing the phase shifting starting much higher up in the frequency band before the frequency roll off starts to have an effect.
If I build one unit, and I have the choice of grabbing this capacitor from the drawer or the other one, I pick the oversized one that will have less of an effect compared to the smaller one.

If I had to produce +100.000 units and make a profit on them, my choice would probably be different.

[sarcasm]
Why is the high-z instrument input on my RME FF800 so noisy ? and why does it sound like crap ? why do I as a hobbyist have to figure it all out myself and build my own to get a proper instrument input ?
That is what the RME engineering team is supposed to do.... not me !
[sarcasm]

I love GDIY !

As far as the human sensory system goes, I'm only an expert on my own sensory system, I don't know what other people are seeing, hearing or feeling, it can sometimes be a bit of a wild guess, but one thing is sure, it does not always line up with mine...

 

[abbey road d enfer]

I understand the frequency roll off is the result of phase cancelation.

No you don't.
Phase cancellation is a quite different matter. It happens when a single source travels through two (or more) different paths that have different propagation times. As a result some frequencies arrive out-of-phase, which results in partial or total cancellation, and other frequencies arrive in-phase and create peaks. That's the well-known phaser effect.

As far as the human sensory system goes, I'm only an expert on my own sensory system, I don't know what other people are seeing, hearing or feeling, it can sometimes be a bit of a wild guess, but one thing is sure, it does not always line up with mine...

One needs to be humble.
Many people think they have extraordinary sensory powers; they end up being audio gurus or audiophools (which is worse, I don't know...).
I'm not saying it's your case, but it's quite easy to fool oneself.
I learned to be humble reading the works of Zwicker a,d Fastl.

 

[PermO]

We did a little testing.... it's pretty good.

Frequency responce, flat, from 1Hz to aprox 450KHz
Noisefloor of the RME FF800 sits at -110dB, we need a better interface to get acurate noise figures from the proto pre, when we removed the proto pre from the FF800 we were looking at the exact same noise spectrum, so yeah it's probably -110dB or lower.
With 1V input at the highest gain setting the sinewave started to distort (clip) a little bit, I don't think distortion at useable gain setting is an issue, this thing will drive anything... hard, without clipping.
With the transformer the bandwith is limited to 45 KHz and there's a bit of ringing.

Played some bass on BIG speakers, compared to his API style pre DI input... again, the proto pre is the clear winner, the other pre was missing the lowest low end and the top of the top end, and when you switch back to the proto pre the other pre sounds flat like a piece of cardboard.

This is a keeper.

 

[PermO]

As it's all about the music.... here's some sounds;


Bass, guitar and Rhodes all recorded with proto pre into AD, no further treatment, no fx, bone dry, just balanced and panned with the midi drummachine on analog mixer.

 

[abbey road d enfer]

As it's all about the music.... here's some sounds;

Nice. Well done!

 

[PermO]

Thank you !

 

[PermO]

Build the same thing again, 7 gain steps, in a small DI style box.
36V dual rail power supply from 4x 9V battery.

Works like a charm, sounds great, it's like a DI box, but it's an active preamp with balanced line output and buffered jack output.

 

[MidnightArrakis]

"All caps are bad !" "Defund the caps !" ... it was not doing much harm, but yes, without the cap it's better.
Here's my drawings;
View attachment 92806View attachment 92807View attachment 92808

PermO: -- Did you ever come up with a fully-complete schematic of your project here, complete with the entire VU-driver circuitry and updated component values? If you only have a hand-drawn schematic version and could share that, I could then send you a CAD-designed schematic version for yourself and anyone else who may be interested in your project. After following this thread, I think you have done a terrific job!!!

You can PM me if you wish so I will definitely see your message, as I may easily miss it if you post it here.

Looking forward to hearing back from you shortly!!! THANKS!!!

/

 

[ubxf]

Would the OPA1656 work as well as OPA2134 ? I just found this application note and it looks like it might be similar

 

[L´Andratté]

Build the same thing again, 7 gain steps, in a small DI style box.
36V dual rail power supply from 4x 9V battery.

Works like a charm, sounds great, it's like a DI box, but it's an active preamp with balanced line output and buffered jack output.

View attachment 113087

Very nice work
If possible, could you post a "gut shot"?
What batteries do you use, 4x9V blocks certainly couldn´t fit incl. circuit and components, could they???

 

[abbey road d enfer]

Would the OPA1656 work as well as OPA2134 ?

It would work even better!

 

[PermO]

Oh wow, the topic came alive !
I'm at my Phone at work now, I will respond later when I'm at home at my computer.

@midnight, I'll be happy to share my designs so you can have a go at it. I've been thinking it would be nice to have a proper circuit board design for this so more People could build this.

@Andre, 9V lithium blocks x4 just fit, plus the circuit and a filter board and a stepped attenuator.

I'll come back to this later, and thanks for showing interest in this project eveyone !

 

[ccaudle]

Would the OPA1656 work as well as OPA2134 ?

Yes, OPA1656 is a newer design made for the same types of applications where OPA2134 was often used (and OPA2604 before that).
One thing you do have to deal with is that OPA2134 is available in DIP, but OPA1656 is only available in SOIC and SOT packages. You can buy SOIC-to-DIP adapter boards if you have an existing PCB that you need to use, or if you want to use perfboard to hand wire a circuit.

 

[PermO]

Ok, here comes the weirdness....

The box used is a Hammond 1590BBS and it's slighty larger comperad to the cheaper lookalikes that will just not fit this contraption.
Batteries only fit back to back, I started out glueing two 9V clips together for easy wiring and you will run into the reset cornerpieces of the box, so clips go on the outside.


There's two layers of circuit board, the bottom one is a powersupply filter with some electrolitic and foil caps in a C-L-C-L-C-R-C order. Two isolated screws connect to the powerswitch, so in the off position you can probe these with a multimeter to check battery status. I've had it running a couple of nights for hours on end and it still reads 36.6V
I have used the more expensive Lithium 9V batteries that also are recommended for smoke detectors, the circuit does not draw a lot of current so they will probably last very long.
The stepped gain switch is a miniature ELMA I found in my drawer I never bought that, but I was shocked how much that costs when I looked it up.

That's all the stuff that goes in the box.
I build on non copper pref board, it's just resin board with holes, I prefer these over copper pads.

This is the as build schematic, lowest setting is for hooking up a synth , the rest of the steps provide plenty gain for any passive instrument / pickup / piezo.
As there is no PSU ground all ground connections connect to pin 1 XLR. It's dead quiet, plenty gain, really punchy tight low end, no dodgy pots in the signal path and no expensive transformers needed.

Just plug in your bass and flip the switch and you can drive that big old vintage compressor hard if you wish, no DI does that.

A pocket sized powered punchy preamp with amazing dynamics and headroom.







Yes, it's cork.

 

[L´Andratté]

Lovely! Built with so much attention to detail. And cork.
And a good reference circuit, need some more piezo preamps soon!
Charge amps are fine for that but not for flexibility of input devices...
Thanks for posting!

 

[PermO]

I was curious, as they felt so differently compared to a tin can alkaline.

 

[PermO]

PermO: -- Did you ever come up with a fully-complete schematic of your project here, complete with the entire VU-driver circuitry and updated component values? If you only have a hand-drawn schematic version and could share that, I could then send you a CAD-designed schematic version for yourself and anyone else who may be interested in your project. After following this thread, I think you have done a terrific job!!!

You can PM me if you wish so I will definitely see your message, as I may easily miss it if you post it here.

Looking forward to hearing back from you shortly!!! THANKS!!!

/

Hi !

I went trough my notes from the first version and yes I will have to update the schematic as I ended up with slightly different resistors on the stepped gain switch.
I will draw an updated schematic this weekend.

The schematic for the powersupply is as build.

The dual channel VU driverboard is from China, they are sold in combination with two VU meters.
I had one faulty meter so I decided to use it in this config as I am still using both channels to read the high gain XLR output or the low gain, front, pedal output.

It has it's own onboard PSU so I feed it AC from the transformer, so the mains PSU is audio only.
So it's hooked up to AC from the transformer directly, it also provides 12V for the VU backlight.

It would be nice to have a circuit board that includes everything for this preamp, I can imagine some people would love to have that.

I added a transformer because I had a couple from an old broadcast installation, the box does not need one, though it might come in handy in a situation on stage where the FOH output and the pedal contraption cause a groundloop.

So yeah, where ever you want to take this...

 

[abbey road d enfer]

This is the as build schematic,

Actually, the input CR make sense only because you had the parts ready. 3.3uF/10Meg results in -3dB @0.005Hz.
This could be shifted about 10 times more without any damage. It would allow using a much smaller cap, like 100nF.
Is it for the same reason you've used 0.22 film caps for rail decoupling, or do you find films to be better than ceramics?

Now, what is this component that looks like a styroflex cap?
I don't see it on the schemo...

 

[PermO]

From the standpoint of electronics engineering / product design I understand your criticism.
Keep it small, cheap and available . And make some profit.

This is not a product, I'm just a curious hobbyist with boxes full of old (broadcast) parts and bits I often get offered for free at the places I work as a free lancer.

My criterium for a " good circuit" is probably completely different then yours, wich is fine by me.

I'm not the average listener, when I listen to music, and the playback is of great quality,... I go blind.
Litteraly, my eyes stop working (grey haze) when sound is really good.
First time that happened it freaked me out completely, but as soon as the music stops my eyesight returns to normal.

So my criterium is, "does the circuit make me go blind ?"

I've been experimenting quite a bit with this, and this phenomenon of going blind is the sole reason for my interest in electronic circuits.
If I did not have this experience I would probably have no interest in building my own audio stuff.

Why does amp A make me go blind and amp B does not ?

I want to know, I want to understand what's happening and electronics classes will not in any way make me wiser in that regard, so I'm on my own, trying to figure it out, and that's what you see here, me, trying to figure sh*t out.

I have not figured it out yet, but to me all components made from carbon / with carbon perform best.
So I use a lot of MKC caps and carbon resistors only.

You are correct in your observation, it is a big styroflex cap and it sits paralel over the input cap, it has no electrical function.

So my component selection makes a lot of sense, to me, I would never advise people to do the same, as it simply won't make sense, this is also the reason I'm always hesitant of posting gut shots, as they always raise questions.

-3dB @0.005Hz, yes the low end sounds really tight and punchy, this thing beats all my active DI inputs and boxes by far, I don't like what transformers do to low end (5 string bass) they always sound sloppy and wooly and wobbly to my ears.

There might be some really good ones out there, but I don't have these.

Hope this helps clarify my standpoint a bit.




Cheers !

 

[abbey road d enfer]

From the standpoint of electronics engineering / product design I understand your criticism.

My comment was not a criticism. It was meant to alert members that would attempt to duplicate your circuit that this huge capacitor is not really necessary, just as using film caps for decoupling rails is not either, when ceramic caps are perfectly adequate.

This is not a product, I'm just a curious hobbyist with boxes full of old (broadcast) parts and bits I often get offered for free at the places I work as a free lancer.

I understand that very well; I believe it was clear when I mentioned that it "make sense only because you had the parts ready".

My criterium for a " good circuit" is probably completely different then yours, wich is fine by me.

I'm not sure we have different criteria...

I'm not the average listener, when I listen to music, and the playback is of great quality,... I go blind.
Litteraly, my eyes stop working (grey haze) when sound is really good.
First time that happened it freaked me out completely, but as soon as the music stops my eyesight returns to normal.

So my criterium is, "does the circuit make me go blind ?"

I've been experimenting quite a bit with this, and this phenomenon of going blind is the sole reason for my interest in electronic circuits.
If I did not have this experience I would probably have no interest in building my own audio stuff.

I don't have this kind of mystic relationship with audition, I've managed to combine my love of music with that of electronics and make a living out of it.

You are correct in your observation, it is a big styroflex cap and it sits paralel over the input cap, it has no electrical function.

If it has no electrical function, why have you put it in? Did you just ask yourself "whet about putting styroflex there?"

-3dB @0.005Hz, yes the low end sounds really tight and punchy, this thing beats all my active DI inputs and boxes by far,

It would be interesting if you changed this capacitor for a much lower value, e.g. 100nf and make a new assessment, or have you already done it and opted for this value after evaluation. I don't really believe you can hear the difference in the response below 5Hz, particularly with musical instruments.

I don't like what transformers do to low end (5 string bass) they always sound sloppy and wooly and wobbly to my ears.

I don't favour transformers either.